In electronic devices, such as personal digital assistants (PDAs), laptop computers, office automation equipment, medical equipment, and car navigation systems, touchscreens are widely used as their display screens that also serve as input means.
There are a variety of touchscreens that utilize different position detection technologies, such as optical, ultrasonic, capacitive, and resistive technologies. A resistive touchscreen has a configuration in which an optically transparent conductive material and a glass plate with a transparent conductive layer are separated by spacers and face each other. A current is applied to the optically transparent conductive material and the voltage of the glass plate with a transparent conductive layer is measured. By contrast, a capacitive touchscreen has a basic configuration in which an optically transparent base material has a transparent conductor layer thereon and there are no movable parts. Capacitive touchscreens, which have high durability and high transmission, are applied, for example, to in-car equipment.
As transparent electrodes (optically transparent conductive material) for touchscreens, optically transparent base materials having optically transparent conductive film made of ITO formed thereon have been commonly used. However, there have been problems of low total light transmittance due to high refractive index and high surface light reflectivity of ITO conductive films. Another problem is that ITO conductive films hare low flexibility and thus are prone to crack when bent, resulting in an increased electric resistance.
For the production of optically transparent conductive materials having an optically transparent conductive film which is an alternative to ITO films, a semi-additive method for forming a conductive pattern, comprising making a thin catalyst layer on a base material, making a resist pattern on the catalyst layer, making a laminated metal layer in an opening or the resist by plating, and finally removing the resist layer and the base metal protected by the resist layer is disclosed in, for example, Patent Literature 1 and Patent Literature 2.
Also, in recent years, a method in which a silver halide diffusion transfer process is employed using a silver halide photosensitive material as a precursor to a conductive material has been proposed. For example, Patent Literature 3, Patent Literature 4, and Patent Literature 5 disclose a technology for making a metal silver pattern on a conductive material precursor having, on an optically transparent base material, a physical development nucleus layer and a silver halide emulsion layer in this order by the action of a soluble silver halide forming agent and a reducing agent in an alkaline fluid. The patterning by this method can reproduce uniform line width. In addition, due to the highest conductivity of silver among all metals, a thinner line with a higher conductivity can be achieved in comparison with other methods, and thus an optically transparent conductive material having a high total light transmittance and a reduced electric resistance can be obtained. An additional advantage is that an optically transparent conductive material obtained by this method has a higher flexibility, i.e., a longer flexing life as compared with an ITO conductive film.
In a projected capacitive touchscreen, two optically transparent conductive materials on each of which a plurality of electrodes are patterned in the same plane are joined together, and the two serve as a touch sensor. While in operation, an operator of a touchscreen usually keeps staring at the display, and therefore if there is a portion in which the total light transmittance is different, the electrode pattern can be recognized (highly visible), causing a problem. In a capacitive touchscreen, an optically transparent material on which electrodes are transversely arranged and another optically transparent material on which electrodes are longitudinally arranged are used in an overlapped state. In this state, the total light transmittance of portions in which upper electrodes and lower electrodes overlap is inevitably reduced, causing the electrode pattern to be seen (highly visible). The electrodes used in a projected capacitive touchscreen comprise relatively large main electrode units in the shape of squares (or diamonds or the like) and relatively small connectors for electrically connecting adjacent main electrode units, and the two kinds are arranged in an alternating manner. Generally, when two optically transparent conductive materials are joined, the smaller connectors are overlapped for the purpose of reducing the areas of different total light transmittance. However, satisfactory effect has not been achieved and further improvement has been desired.
To address this problem, for example, Patent Literature 6 and Patent Literature 7 disclose a conductive sheet in which middle-sized lattices having a pitch n times as long as that of small lattices that constitute large lattices that serve as main electrodes are arranged in a zigzag manner. This technique improves the overlapping between the upper electrodes and the lower electrode. However, in this case, minute blank spots which correspond to gaps between the vertexes of the upper electrode pattern and also to those of the lower electrode pattern exist. As a result, surface roughness of the touchscreen can be felt, and thus further improvement has been desired.